1. Field of Invention
The present invention is related in general to a ballast, and more particularly, to a ballast of fluorescent lamps.
2. Description of Related Art
Fluorescent lamps are the most popular light source on the market today. By improving the efficiency of the fluorescent lamps and/or providing the dimming control will offer significant savings in energy. Therefore, in recent developments, the capability for improving the efficiency and power savings for fluorescent lamps is a major concern. In addition, prolonging the fluorescent lamp's lifespan is also important for reducing environmental pollution. The proper starting and operating of the lamp is able to provide a longer lamp lifespan. Prior to ignition, the cathode filaments of the lamp electrodes should be preheated to an appropriate emission temperature. During normal operation, the cathode filaments should be maintained at an emission temperature. Furthermore, the glow discharge should be prevented because it will cause the filaments to wear out. The glow discharge is occurred when the filaments are being preheated and during an instance of higher voltage across the lamp. Therefore, the lamp voltage should be limited to within controlled ranges during the preheating interval. In order to completely eliminate the glow discharge, an additional filament heating circuit may be needed. However, such an approach has led to higher costs.
FIG. 1 shows a conventional electronic ballast with a series resonant inverter. The half-bridge inverter includes two switches 10 and 11. The two switches 10 and 11 are interchangeably switched on and off under a 50% duty cycle at the desired switching frequency, which can be controlled to fulfill the requirements during both starting and normal operations. The resonant circuit is formed by an inductor 13, a capacitor 14 and a fluorescent lamp 15. The fluorescent lamp 15 is coupled in parallel with a capacitor 16. The capacitor 14 is operated as the starting circuit. During the preheating stage, the lamp voltage can be maintained low by deliberately operating the ballast at the resonance frequency of the starting circuit. After the cathode filaments have been preheated to an appropriate emission temperature, the ballast frequency is adjusted to generate the required high ignition voltage. Once the lamp is running, the frequency is controlled to produce the required lamp voltage. The drawback of the aforementioned circuit is the having of a high glow current. During the preheating interval, the lamp voltage is determined by the switching frequency of the switches 10, 11 and the resonant frequency of the starting circuit. Once the ballast has been switched on, a resonant current is flowed through the capacitor 16 and the filaments for preheating. At this time, the lamp voltage is simultaneously produced on the lamp, which causes an inevitable glow discharge. Another disadvantage of the aformentioned circuit is the reduced control of efficiency during normal operation. The resonant frequency is varied according with a change of the parasitic devices of the fluorescent lamp. The parasitic devices of the fluorescent lamp, such as equivalent capacitance, for example, are changed in response to a change in temperature and the age of the lamp. Furthermore, the input voltage, the resonant frequency, and the characteristics of the fluorescent lamp affect the power consumption of the lamp.
The objective of the present invention is to provide a flyback ballast with improved efficiency. Another objective of the present invention is to eliminate the glow current, and thus prolonging the lamp lifespan.